This invention relates to a speech decoding apparatus and, in particular, to a speech decoding apparatus for use in a radio telephone apparatus of a digital communication system.
A radio telephone apparatus of a digital communication system comprises a coding and decoding apparatus (CODEC) for interconversion between an analog speech signal and a digital speech signal. As a coding system adopted in the coding and decoding apparatus to convert the analog speech signal into the digital speech signal, use has generally been made of a PCM (pulse code modulation) system for coding Instantaneous voltages at sampling time instants, a .DELTA.M (delta modulation) system for coding a difference between sampling time instants, or the like.
In the coding and decoding apparatus of the type described, a correlation exists between an error in a reception signal and a decoded waveform. In this connection, a user of the coding and decoding apparatus can audibly recognize deterioration of the quality of a transmission path in a proportional relationship. Accordingly, the user has an option to maintain or terminate communication in dependence upon the deterioration of the reception signal.
On the other hand, a coding and decoding apparatus of a novel type has recently been developed and put into practical use. For example, there is known a speech coding and decoding apparatus (SPEECH CODEC) in which speech information alone is transmitted and a characteristic component of a speech signal alone is coded in order to reduce a transmission rate in a radio channel area and to narrow a modulation bandwidth.
The speech coding and decoding apparatus is constructed in accordance with a standard "STANDARD 27A" established by RCR (Research & Development Center for Radio System) in Japan.
The speech coding and decoding apparatus has a high signal compression ratio. Accordingly, a speech reproduction characteristic after decoding is extremely deteriorated when a code error occurs in the transmission path. In this connection, when the speech coding and decoding apparatus is used, an error detection/correction code is added to an encoder output to assure the quality of the transmission path.
Generally, an error detection capability is greater than an error correction capability. Inasmuch as a signal error rate in the transmission path is within the correction capability, a decoder is supplied with a corrected signal. When the signal error rate is increased to exceed the correction capability, an error detection function alone is operable. In this event, false codes are discarded and are not supplied to the decoder. Alternatively, a decoded output signal is chuted so that no output is produced.
When the quality of the transmission path is deteriorated, the user of the speech coding and decoding apparatus can hear a normal speech sound before a certain level of deterioration is reached. Thereafter, a communication channel is abruptly interrupted (silent). This results in an embarrassment of the user.